In recent years, liquid crystal display devices (LCDs) have expanded applications, and LCDs are applied even to various displays used outdoors. The applications of LCDs have been expanded, for example, to instrument panels of automobiles, ships, and airplanes; mobile devices such as automobile-mounted navigation systems, digital cameras, mobile phones, and personal computers; and digital signages used in buildings, supermarkets, and other facilities.
An LCD performs display by causing a liquid crystal panel, in which a liquid crystal cell is sandwiched between two polarizers, to transmit or block the light from outside or the light generated by a light source such as frontlight or backlight. As the backlight light source, it is ordinary to use fluorescent tubes such as cold-cathode tubes or hot-cathode tubes. The spectral distribution of fluorescent tubes such as cold-cathode tubes or hot-cathode tubes shows an emission spectrum having two or more peaks. The combination of colors in such a discontinuous emission spectrum provides a white light source. Meanwhile, the applications of light-emitting diodes, which consume low power, have been studied in view of energy conservation. In particular, white light-emitting diodes (white LEDs) have a more continuous and wider emission spectrum than that of fluorescent tubes, and also have an excellent luminous efficiency.
Incidentally, there is a case where, in an environment such as outdoors in the strong sunlight, an observer views an LCD while wearing sunglasses having polarization properties to eliminate the glare. In this case, the observer views, through polarizers, linearly polarized light emitted from the LCD. Therefore, the screen cannot be viewed depending on the angle between the absorption axis of a polarizer included in the LCD and the absorption axis of a polarizer such as sunglasses.
To solve the above problem, for example, Patent Document 1 proposes a method for performing depolarization by layering a retardation (one-quarter wavelength) film obliquely on the surface of an LCD to convert linearly polarized light into circularly polarized light.
In addition, Patent Document 2 proposes the use of a highly birefringent material, such as calcite or synthetic quartz, as a depolarizing device to solve the above problem. This solution makes use of the following properties: If a highly birefringent material such as calcite or synthetic quartz is inserted between crossed polarizers (i.e., crossed-nicols), the light transmitted through the highly birefringent material having a great retardation (e.g., a retardation of higher than 100,000 nm) becomes light having various wavelengths. Thus, the resulting light interferes with itself due to the various wavelengths and presents white light.